A double-integration type A/D converter performs A/D conversion by integrating an input voltage for a specified time, integrating a reference voltage whose polarity is opposite to that of the input voltage, and counting the time it takes for an output of an integration circuit to reach a predetermined level.
A conventional technology employs a double-integration type A/D converter which eliminates a zero point error by an offset voltage of an integrator using an operational amplifier. In this type of an A/D converter, integration is performed twice in the same integrator in order to eliminate the zero point error by the offset voltage of the integrator.
Another conventional technology uses an A/D converter in which current having a polarity opposite to that of an input voltage is discharged by a constant current circuit connected to an input side of an integrator during an input voltage integration period or a reference voltage integration period. Thus, this technology may achieve reduction of cost by reduction of A/D conversion time and reduction of the number of bits of a counter circuit.
A still yet another conventional technology adopts an integral type A/D converter which converts an output voltage of a detection circuit such as a bridge circuit for measurement of distortion (e.g., Wheatstone bridge circuit) or the like into digital data. Thus, this technology may achieve fast A/D conversion processing while appropriately eliminating unnecessary components including hum components caused by AC power.